Angiogenesis plays a pivotal role in several important disease processes as well as in normal physiology. It is widely anticipated that modulation of angiogenesis (inhibition in tumors, stimulation in vascular insufficiency) will provide important therapeutic benefit. Many different cytokines and growth factors express angiogenic activity, of these VEGF-A stands out because of its potency, selectivity for vascular endothelium, and its consistent overexpression in malignant tumors and in other clinical conditions in which angiogenesis plays an important role. VEGF-A acts selectively (though not exclusively) on endothelial cells (EC) by means of two high affinity receptor tyrosine kinases VEGFR-1 and VEGFR-2/Flk-1. Both of these receptors are expressed at increased levels by EC during development and in pathophysiological angiogenesis. Most of the endothelial cells express both of the receptors and both of them can homodimerize upon binding to VEGF-A; therefore, it is difficult to comprehend the molecular function of the individual receptor in the presence of the same ligand. The proposed study aims to dissect the functional aspects and sole responsiveness of these receptors for VEGF-A mediated signaling in EC. Chimeric receptors of both VEGFR-2 and VEGFR-1 and their respective mutants will be utilized to study signaling pathways responsible for VEGFR-2 and VEGFR-1 in vascular endothelial cells. Aim 1 will focus to reveal the molecular function and dissect the signaling pathways for proliferation versus migration channeling through VEGFR-2/Flk-1. We will also define the receptor(s) responsible for endothelial cell sprouting and its subsequent signaling pathways. In Aim 2, investigation of the functional aspects of VEGFR-1 in endothelial cells will be performed. Furthermore, examination of the inhibitory role of VEGFR-1 for the VEGFR-2 function(s) and the pathways necessary for the inhibition will also be demonstrated. In addition, it will be tested whether VEGFR-1 has any functional relationship with neuropilin-1, a new VEGF-A receptor of unknown function, particularly in EC migration. In Aim 3, the data from Aims 1 and 2 will be utilized to evaluate the signaling pathways between normal versus tumor-induced angiogenesis. A novel protein delivery system will be utilized or retroviral mediated genetic manipulation will be carried out to inactivate the target molecule(s) in normal as well as tumor-induced angiogenesis. By targeting the same signaling components in normal as well as tumor-induced angiogenesis, we will get a better picture and make a better comparison between these two events. The proposed study thus will delineate the individual role of the receptors in VEGF-A-mediated signaling and will also shed new light on the molecular mechanisms of angiogenesis. Taken together these experiments are likely to identify new therapeutic targets in order to combat angiogenesis in tumors and also in other disease processes.